Add NUC126 support

[FlamingCupcake]

Resolves #337

This is a draft of support for the NUC126 family of MCUs.

The implementation is largely based upon the existing implementation for the NUC123 family.
It also contains:

• Drivers for ST, USB and PAL
• Demos for the USB and PAL drivers (currently untested, EV board on the way)

Current version should be working (🤞 no jinx) even though it has some TODOs (which are just nice-to-haves) but might not fit the code style.

[fpoussin]

Let me know when you are ready to merge. In the meanwhile, could you add it to the CI workflows?
Thanks

[FlamingCupcake]

Hi, just dropping an update. I got my devboard recently and am working on squashing bugs.

I'll see to adding it to the CI. If there's anything else that you'd like me to add, please let me know.

[kovmir]

Can this be merged?

[doombadroid]

Hi @FlamingCupcake, thanks for this port. I've been running it on real hardware (a NUC1261SG4AE in a Ducky One 3 keyboard, with QMK). It needed a few fixes to build on the current chibios-21.11.x and to bring USB up, so I've opened #454. It keeps your commit unchanged, with your authorship, and adds the fixes, updated demo configs and the CI build @fpoussin asked for on top. If you'd rather fold them into this PR, I'm happy to close mine.